In power converters, losses appear as none of the components have ideal characteristics. The losses introduce heat in the power circuitry, which apart from consuming energy introduces thermal strain to all components, reducing the life span.
It is desirable to increase the frequency of operation of power converters as the generated output then can be more exactly controlled. Increasing the switching frequency leads to lower switching ripple, smaller component values which in turn leads to a more compact, light-weight and cost-effective implementation of the invention. Further, lower switch ripple allows for potentially lowered EMI, which in turn goes closer to the goal of a non-disturbing switch. Still further, having a high switching frequency allows for higher frequency currents to be generated by the power converter, widening the range of suitable applications for the converter.
However, increasing the frequency also increases switching losses, as most losses appear on switching cycle basis. Forcing the semiconductor to commutate while a current is running through it or when there is a voltage difference over it, requires energy which must be supplied to the semiconductor. Thus, reducing the current through the semiconductor or the voltage thereover reduces the total power input for the switch and thus the total power input to the system.
One way of reducing the losses over a particular switch is to add a resonant component to the circuitry in which a current is generated by an inductive element, by the discharge of a capacitor. A circuit employing this technique is known as a resonant converter. The use of a resonant converter enables soft switching, which reduces the energy consumed in the switching operation. There are generally two types of soft-switching: zero-voltage switching and zero-current switching. Zero-voltage switching involves minimizing the voltage difference over the switch prior to commutation, whereas zero-current switching involves minimizing the current therethrough prior to switching. For true zero-voltage switching to be possible, the resonant circuit must adjust the voltage on one side of the switch just right, such that no potential difference over the switch exists.
Generating less EMI noise is an important goal in its own right. In applications where the converter or inverter is connected directly to the grid, EMI noise can cause problems which are normally solved by employing EMC-filters. EMC-filters must be placed in series with the converter, thereby handling the full current capacity. By minimizing the EMI, EMC filters can be eliminated from the converter design.